The purpose of this project is to clarify the mechanism of cell transformation by examining the genetic and physiological factors which affect cell transformation induced by chemicals. This project may also lead to development of an optimum system to study the mechanism of cell transformation as well as to obtaining an assay system for the assessment of human risk from environmental carcinogens. The structure-activity relationship of tumor promoters is being examined by using new promoters, aplysiatoxin (AP) and debromoaplysiatoxin (DBAP), with transformation of Balb 3T3A31-1-1 cells as an indicator of promoter potential. AP strongly enhances carcinogen-initiated transformation, whereas DBAP does not. The new factor which abolishes DBAP effects but not AP was found in serum which was used for transformation experiments, and was purified and identified as a protein of approximately 72,000 mol. wt. The lack of activity of DBAP in enhancement of transformation was ascribed to its high inactivation by the protein factor in serum. This transformation assay system correctly predicted the weak promoting potency of DBAP in vivo. Studies of the inhibitory effects of dexamethazone on cell transformation indicated the involvement of EGF-receptor down-modulation mechanisms in the cell transformation. A culture model system for cocarcinogenesis was developed using catechol, a mouse skin co-carcinogen, and Balb 3T3-A31-1-1 cells.